The presence and management of water within proton exchange membrane fuel cells (PEMFCs) are key factors in the performance, durability, and cost associated with the resultant fuel cell systems. Dependent upon the specifications of the particular system, a certain minimum amount of water is required to maintain hydration of the fuel cell membrane and to facilitate efficient proton conductivity in the polymer electrolyte. Moreover, liquid water is a preferable reactant at the anode under fuel starvation conditions that often can occur during fuel cell startup and shutdown; otherwise, carbon may react in substituting hydrogen to sustain the power requirement, which leads to catalyst carbon corrosion and fuel cell degradation. While a certain level of water is required for efficient operation of the fuel cell, excess water accumulation in the system's gas channels and porous electrodes, including the catalyst and gas diffusion layers, can lead to a variety of performance and durability problems. Such problems can include voltage loss at high current density due to reactant gas transport limitations, voltage instability at low current density, unreliable startup under freezing conditions, and corrosion of the carbon catalyst support due to hydrogen starvation. In light of these requirements and drawbacks, water management within PEMFC systems is integrally important to their operation.